1. Field of the Invention
Exemplary aspects of the present invention generally relate to a developing device, a process cartridge including the developing device, and an image forming apparatus including the process cartridge.
2. Description of the Background Art
Conventionally, a developing device equipped with a developing sleeve serving as a developer bearing member having a surface including a plurality of recessed portions for assuring stable developer conveyance is known as a developing device used in image forming apparatuses such as a copier, a facsimile, a printer, and a plotter.
For example, as disclosed in Japanese Patent Unexamined Application Publication No. 2001-134069, a developing device includes a developing sleeve having a plurality of grooves serving as recessed portions extending in an axial direction and provided on a peripheral surface in a circumferential direction of the developing sleeve.
Another known developing device includes a developing sleeve, the peripheral surface of which is sandblasted or otherwise surface-finished so as to have a coarse surface including a plurality of recessed portions.
A developing sleeve including the plurality of grooves or the recessed portions formed on the peripheral surface by sandblasting or other surface finishing is able to consistently bear the developer, thereby stabilizing an amount of the developer that is borne on the peripheral surface of the developing sleeve and passes a regulating position at which a developer regulating member further regulates and refines the amount of developer.
However, the following problems may occur in a developing sleeve, the surface of which either includes the grooves described above or is subjected to surface finish such as sandblasting.
FIG. 14A is a cross-sectional view illustrating a developing sleeve and a photoreceptor, according to a related art, and FIG. 14B is a side view illustrating a supporting plate including shaft bearings illustrated in FIG. 14A, according to a related art.
With reference to FIGS. 14A and 14B, according to Japanese Patent Unexamined Application Publication No. 2004-21122, for example, relatively large amounts of developer are borne at both end portions of a developing sleeve 750 in a width direction of an image creating region of the developing sleeve 750, and are transported to a developing region (nip portion) of the developing sleeve 750 across from a photoreceptor 800 serving as an image bearing member.
Consequently, developer density rises at each of both ends of the image creating region in the width direction of the developer sleeve, thereby intensifying pressure on the developer between the photoreceptor 800 and the developing sleeve 750.
As a result, the developer may firmly stick to the surface of the developing sleeve and/or the developer may drop from each of both ends of the developer sleeve. When the developer firmly sticks to the surface of both ends of the developer sleeve, problems such as peeling of the surface layer of the photoreceptor, background contamination of an image, banding due to an increase or fluctuation in operational load, and cleaning failure may occur in the image forming apparatus.
In particular, according to Japanese Patent Unexamined Application Publication No. 2004-21122, when a developer consisting of small-diameter particles is used and/or a developing gap between the photoreceptor 800 and the developing sleeve 750 is reduced in order to achieve better imaging quality, the developer is likely to stick firmly to each of both ends of the developing sleeve.
In an attempt to reduce, if not prevent entirely, the problems described above, according to Japanese Patent Unexamined Application Publication No. 2004-21122, an outer diameter of the developing sleeve is deliberately made smaller at each of both ends thereof. That is, the outer diameter of the developing sleeve is less than that of a center portion of the developing sleeve, so that the overall shape of the sleeve is such that the sleeve bulges slightly at the middle and tapers toward both ends.
However, because the outer diameter of the developing sleeve is reduced at both ends, a difference in height or a step is formed at each of both ends of the developing sleeve. The step portion at both ends may damage the photoreceptor when the step portion comes into contact with the photoreceptor. Furthermore, upon assembly, mounting, and transport, the developing sleeve may contact the photoreceptor, thereby damaging the photoreceptor and thus causing image failure.
In order to achieve high image quality and reduce irregular image density, a developing space or gap Gp between the developing sleeve 750 and the photoreceptor 800 needs to be relatively small, and the size of such small gap needs to be maintained accurately and consistently.
In order to obtain such a gap, in the related art image forming apparatus, after installation of the developing sleeve or the developing unit in the image forming apparatus, a pair of plate members (similar to plate members 101 and 102 in FIG. 4) is affixed to both front and rear ends of the developing sleeve and the photoreceptor so as to fix the positions of the developing sleeve and the photoreceptor.
However, there is a problem with such an approach, in that the developing sleeve may contact the photoreceptor before the plate members are installed at both ends, thereby damaging the photoreceptor.
Consequently, as illustrated in FIG. 15, which is a conceptual diagram illustrating a plan view of a method for positioning the developing sleeve and the photoreceptor of FIG. 14 according to the related art, a main frame 210 of the image forming apparatus main body is typically provided with a shaft 211 which is inserted into an insertion hole 800b of the photoreceptor 800 so as to fix the photoreceptor 800 in place and maintain the gap Gp between the developing sleeve 750 and the photoreceptor 800 at a predetermined size. A more detailed description of this arrangement follows.
In FIGS. 14 and 15, a PCU unit 500 including a developing device equipped with the photoreceptor 800 and the developing sleeve 750 is illustrated. The PCU 500 is detachable from the main frame 210 of the image forming apparatus.
As illustrated in FIGS. 14 and 15, the photoreceptor 800 is a hollow-type photoreceptor. Shaft bearings 280 rotatably support a shaft 752 provided to both end portions 751 of the developing sleeve 750. The end portions 751 of the developing sleeve 750 are provided outside the image forming region of the photoreceptor 800. The outer diameter of the end portions 751 is smaller than the outer diameter of the center portion of the developing sleeve 750.
Shaft bearings 281 rotatably support the shaft 211 which is inserted into the insertion hole 800b of the photoreceptor 800. The shaft bearings 280 and the shaft bearings 281 are provided to supporting plates 105 and 106 so as to position the developing sleeve 750 and the photoreceptor 800.
As illustrated in FIG. 15, the PCU unit 500 is moved in a direction indicated by a hollow arrow to be mounted on the main frame 210 of the image forming apparatus so that the shaft 211 is inserted into the shaft bearings 281 and then inserted into the insertion hole 800b of the photoreceptor 800.
A problem arises here, in that a biasing force of a spring, not shown, pressing a tip of a cleaning blade 275 (similar to a cleaning blade 75 shown in FIG. 12) against the surface of the photoreceptor 800 causes the photoreceptor 800 to move to the left as indicated by a small solid arrow. Consequently, the shaft 211 may be misaligned, by an amount indicated by reference numeral 215, for example.
In order to insert the shaft 211 into the insertion hole 800b of the photoreceptor 800, the PCU unit 500 needs to be moved laterally left and right to align the shaft 211 with the insertion hole 800b. Consequently, inertia or the like occurs in the photoreceptor 800 so that a predetermined gap Gp between a surface 800a of the photoreceptor 800 and the surface of the developing sleeve 750 cannot be secured, and thus, an edge portion of the step portion of the developing sleeve 750 having a smaller diameter than the center portion thereof may contact the surface of the photoreceptor 800, thereby damaging the surface of the photoreceptor 800.
Still further, upon transport of the image forming apparatus in a state illustrated in FIG. 4, for example, when vibration, shock, or the like is applied, there is a possibility that the developing sleeve 750 and the photoreceptor 800 may contact each other. This is especially the case when the gap Gp is configured to be relatively small so as to achieve high quality imaging, and as a consequence inadvertent contact between the developing sleeve 750 and the photoreceptor 800 occurs more readily.
Still further, when packing a process cartridge, a container is likely small so that the photoreceptor 800 may be easily affected by vibration and shock during transportation. It is to be noted that the hollow-type photoreceptor 800 without a shaft tends to be affected easily by vibration and shock.